A novel method has been developed to microspectrophotometrically determine the oxygen saturation of blood in very small arteries and veins. This will be tested against the Van Slyke method. By combination of this technique with a flow measurement using microspheres it will be possible to determine regional tissue oxygen consumption by the Fick method. This is the first method developed to measure regional oxygen consumption of small portions of any tissue. The technique will also be tested against whole tissue measurements. The technique will be applied to the heart to measure regional tissue oxygen consumption of the ventricular walls, which will be studied with special regard to superficial and deep layers and base and apex oxygen consumption in the anesthetized open chest animals. The action of various stresses that will alter total myocardial oxygen consumption will be studied to determine their regional effect. The effect of increases in heart rate by atrial pacing will be studied. Reduction in coronary blood blow through one or more arteries will be used to learn its effect on regional oxygen consumption. The effect of increases in blood pressure by aortic constriction on oxygen will be determined. These changes can give much information on the effects of several cardiovascular diseases. The role of drugs used for the treatment of angina pectoris and related disorders will be examined with regard to regional myocardial oxygen consumption. This microspectrophotometric technique once established can be used to explore any tissue in addition to the heart. There are other organs where this technique is especially applicable such as the kidney and brain, where known regional differences in oxygen consumption exist. It can be used to explore both healthy and diseased organs.